Prolactin (PRL), a pituitary polypeptide hormone, has pronounced physiological effects on normal and neoplastic growth and on differentiation. Evidence has accumulated to suggest that PRL may be the initial fetal and neonatal growth hormone in mammals. Moreover, recent studies showing a stimulatory effect of PRL on tumor growth have substantiated a broad role of PRL in neoplasia. We have also demonstrated that at least one possible site of action of the potent immunosuppressive drugs, cyclosporine and didemin B, may be their ability to antagonize PRL-receptor interactions on human and murine lymphocytes. Therefore, a precise understanding of the mechanism(s) of PRL action may suggest new therapeutic targets for pharmacological manipulation of normal and malignant cell growth as well as immune processes. We propose to investigate the mechanism(s) of PRL action in two in vitro paradigms of PRL-stimulated mitogenesis: (1) the Nb2 node lymphoma cell line (absolutely dependent upon PRL for mitogenesis) and (2) specific subpopulations of murine splenic lymphocytes in which PRL serves as a co-mitogen. Preliminary evidence suggests that PRL transmembrane signal generation involves stimulation of events associated with protein kinase C (PKC) activation. Using these systems, we will pharmacologically assess the relationship between PRL-stimulated ornithine decarboxylase (ODC) induction (a G1 marker of cell cycle), proliferation, and PKC activation. This will include assessment of PKC activation, polyphosphoinositide turnover, ionic fluxes and calmodulin interactions. We have also established the presence of benzodiazepine receptors on Nb2 lymphoma cells and a direct relationship between agonist stimulation and ODC induction and proliferation. Data are presented that characterize benzodiazepine receptors of Nb2 lymphoma cells as peripheral in nature. These studies will be extended to lymphocytes. Finally, since proto-oncogene activation has been linked to cellular proliferation, we will examine the relationship between PRL- stimulaed proto-oncogen activation and cell division cycle gene expression in these two models of PRL-provoked cell division. By employing a multidisciplinary approach, pharmacological, biochemical, and genetic, it is hoped that defining the mechanism of PRL-stimulated normal and malignant cell growth may ultimately allow us to target drugs to inhibit specific pathological consequences of PRL action.